Computerized color graphics systems and electronic printing systems are known in the art. Typically, they enable a user to produce a color image—any visual two-dimensional pattern including one or more text, graphic line art elements, continuous tone image elements, and so forth—and from that image to produce a representation that can be printed using a color reproduction system, for example, by producing color separation plates for offset printing. There has been much effort in the past to develop ways to accurately predict the appearance of such an image when printed on a substrate, for example on paper or film, using a number of colorants, such as inks, the prediction carried out without actually printing the image. Similarly, there has been effort to simulate the appearance of such an image without printing the image.
It is desirable to be able to calculate accurately how a picture will look when one prints an image, including a halftoned continuous tone color image, with a certain printing technique on some certain type of substrate using a certain set of colorants (e.g., inks). A method of predicting the color appearance can be used for example to display a simulation of the color appearance on a computer display, or to print a simulation of the color appearance on a more easily accessible and cheaper printer as a proof of what is to be printed finally in production.
It is desirable to do this for both reflection printing on an opaque substrate, and transmission imaging on a transparent substrate.
In order to so predict the appearance of overprints, it is advantageous to spectrally characterizing colorants by parameters that are substantially invariant for all substrates of the same type, and independent of the substrate color, in that, when there is no interaction between colorant layers, one can treat a substrate with one or more colorant overprints on it as a raw colored substrate of the same type but a different color, that different color dependent on the one or more colorant overprints.
The Parent Patents define such colorant parameters and further provide a method for determining the colorant parameters by carrying out some experiments. The colorant parameters are applicable for printing colorants on top of each other with halftoning. The Parent Patents further describe how to determine the spectrum of an overprint of halftoned colorants using the colorant parameters, and also how to obtain the color of an overprint in some color coordinate system, e.g., a device independent color coordinate system such as CIE Lab.
The use of these parameters is described in U.S. Pat. No. 5,933,578 for cases that include when there is little or no interaction between the layers. The method and apparatus of U.S. application Ser. No. 09/361,435 are applicable in cases that include when the colorant parameters of one colorant may be influenced by the other colorants being printed.
Although the methods of the Parent Patents are suited to predicting the overprints of colorants using some spectral colorant parameters, there is still room for improvement. A printing device is often characterized by a device profile. Modem color management techniques such as COLORSYNC™ (Apple Computer, Inc., Cupertino, Calif.) and the methods promoted by the International Color Consortium (ICC, see http://www.color.org) use such profile. A popular profile format is the ICC profile. Such device profiles includes a lookup table providing mapping between some overprint values and colors in some device independent space, e.g., CIE Lab. The normal use of an output table is with multidimensional interpolation to determine the colorant values for some given color specified in the device independent space. ICC profiles do not typically contain any spectral data or any spectral colorant parameters. They are however common in industry, that is, they are often used to predict the color rendition of CMYK printing devices.
It is often required in a job to not only use CMYK colorants, but also to overprint some other colorants, such as PANTONE® colorants or other special inks. Such occurs, for example, in packaging, where logos or items of linework are combined with continuous tone images. The logos or linework elements are often printed in PANTONE® inks and the CT images are mostly printed with CMYK inks. Since overprints of the special ink with any of the CMYK inks have not been measured, there is a problem to determine the color of such overprints. A typical device profile is specified for determining predicting the color of the CMYK parts in a job. There is presently no suitable method for using the profile for determining the color of the overprints of the CMYK of the profile with additional inks.
A first possible solution to this problem is to use a model such as determining the colorant parameters of the Parent Patents. However since the colorant parameters of the CMYK inks are not present in the device profile we have to use some colorant parameters of some CMYK inks, known in the Color Management System. Usually these inks are quite different from the inks of the device profile. This leads to very inconsistent or unstable behavior in the Color Management Systems, since for instance a color consisting of any combination of CMYK will be predicted using the device profile, whereas a color consisting of for instance cyan (C) and a certain percentage (even if this is only 1%) of a special ink will be predicted using some colorant parameters for some cyan ink known in the system and the colorant parameters of the special ink. This may cause the CMS to behave in an unstable manner, because the two colors are calculated using a different path and the result of both paths can be quite far from one another, even if no special ink is involved in the calculations. One aspect of the invention aspect is dealing with the inconsistency issue and stability issue. Another aspect of the present invention is providing device profiles, such as ICC profiles, using the colorant parameters as have been described in one or both of the Parent Patents.
A second possible solution to the described problem, which would not exhibit this unstable behavior, is to make a prediction of the overprinting color by applying a mixing formula in for instance a CMYK, CIE related or RGB color space. With such a method, the color of the cyan would be predicted using the device profile, the color of the special ink would be predicted using its colorant parameters, as if it would not be printed in combination with the cyan, and the color of the overprint would be predicted by some mixing formula in a CMYK, RGB or CIE related color space. While this solution may not have the disadvantage of the “unstable” behavior, the resulting color may be quite far off from the reality, since mixing inks in CMYK, RGB or CIE related color spaces is known to often be inaccurate. One aspect of the present invention deals with finding a solution to this accuracy problem.
Needed is a method and apparatus that solves the aforementioned problems and combines the concepts and technology described in the Parent Patent with the technology of device profiles.
The order of printing of inks is often important. For instance if the inks would be printed in the order C-M-special ink-Y-K, we need to make sure this order is taken into account in the calculation of the overprint color, especially when the special ink has some opacity, since then it will “mask” the cyan and magenta ink, being printed underneath. The extent of the masking is dependent on the degree of opacity of the special ink. If one obtained the color of the C-M-special ink-Y-K ink combination using the device profile for obtaining the color of the CMYK inks and the colorant parameters of the special ink to obtain the color of the special ink, as if the CMYK and the special ink were not to be printed in combination, if then the color of the entire ink combination is obtained by applying some mixing formula in some color space, the opacity of the special ink would probably not be incorporated correctly and the resulting color may be quite far off from the real color.
It is another aspect of the present invention to solve the aforementioned printing order problem, which leads to inaccurate prediction of color. Needed is a more detailed description of how this problem is solved.
By accurately calculating we mean a maximum deviation between predicted color and actual color of the order of 5 CIELAB Delta E units, and an average deviation of about 2 CIELAB Delta E units.
The Prior Art for Predicting the Color of Overprints
Printing may be carried out using halftoning, also called screening, which is the process of creating the illusion of a continuous tone (“CT,” “contone”) image using an output (e.g., printing) device capable only of binary output (ink deposited or not deposited at any location on a substrate). For color printing, several images (“separations”) are produced in the primary colorants (typically inks) used to print in color, and printed over each other in a press. For typical four color printing, four images are produced in cyan (“C”), magenta (“M”), yellow (“Y”) and black (“K”), and each of these images are halftoned. Usually, digital halftoning is used together with an imagesetter, laser printer, ink jet printer, digital film recorder, or other recorder output device.
Prior to our Parent Patents (U.S. Pat. application Ser. No. 09/361,435 and U.S. Pat. No. 5,933,578), various methods were known for calculating the color resulting from superimposing a set of colorant layers on a substrate. These methods can be divided in two groups. The first group is characterized by requiring printing and measuring a relatively large number of overprints of the colorants. That is, this group includes methods that for a particular set of colorants, a particular printing technique (e.g., offset printing on a particular imagesetter), a particular substrate type (e.g., paper, or film for a photographic transparency or print, textile, sheet of plastic, etc.), a particular substrate color (e.g., the color of the paper, or of the transparent film in the case of a transparency, or of the textile, or of the plastic sheet, etc.) and a particular order of printing the colorants, involve printing a relatively large number of overprints of the colorants in the set, for example as patches. These patches are measured with a spectrophotometer or calorimeter and the measurements are used to calculate any overprint of colorants in the set using mathematical techniques, for example, interpolation. If carried out well and carefully, these methods can lead to accurate results. While these techniques can produce accurate results, and also work for halftone images, there are several drawbacks with such methods. One is that a large number of color patches of overprints need to be made. For example, the IT8.7.3 chart (American National Standards Institute [ANSI] Committee IT8 for Digital Data Exchange Standards) contains nearly a thousand patches for a four-color output. Hence it is very difficult to characterize sets of more than four colorants, for example printing with six or seven colors (“HI-FI” printing including PANTONE® Hexachrome from Pantone, Inc., Carlstadt, N.J.). There also are applications where inks other than cyan, magenta, yellow and black need to be used. Another drawback is that a set of patches will only be useful for accurately calculating overprints using the particular colorants and the particular colorant printing order used in the patches. Changing one colorant in the colorant set or changing the order of overprinting typically requires redoing the whole job of printing the set of patches.
An additional problem occurs when using such characterizations with more than four inks in a typical modem color management workflow, such as the ICC workflow. The ICC standard typically uses look up tables and interpolation to predict the CIELAB values of a particular output device. The number of nodes in these lookup tables increases exponentially with the number of inks, and the amount of computer resources becomes too high to be feasible.
Also included in this first group of known methods are those that use the well known Neugebauer equations to predict the color of an overprint of colorants, and their derivative based on Yule-Nielsen and spectral Neugebauer equations (see H. Neugebauer: “Die theoretischen Grundlagen des Mehrfarbenbuchdrucks”, Zeitschriftfur wissenschaftliche Photographie, Photophysik und Photochemie, Band 36, Heft 4, April 1937, Arney:” Modeling the Yule-Nielsen Halftone effect,” Journal of imaging science and technology, vol. 40, No. 3, pp. 233–238, June 1996, and Rolleston: “Accuracy of Various Types of Neugebauer Model”, IS&T and SID's Color Imaging Conference: Transforms and Transportability of Color, pp. 32–37, 1993. These Neugebauer-like models still need the knowledge of the color of the overprints of the primary inks, so still requires measurements overprint combinations of the colorants and the gradation steps of the colorants. Also, Neugebauer equations-based methods are known not to produce accurate results.
A second group of prior art methods are those that determine spectral characterizations of individual colorants that can be used for predicting the color of overprints of so-characterized colorants. These methods, for a fixed substrate and printing technique, involve making one or more printouts of each colorant on one or more substrates, measuring the prints, and out of this data extracting a set of one or more parameters for each colorant that can be used to calculate an overprint of each colorant. These methods thus have the advantage of not requiring producing a large set of overprints. One such prior art method uses the two-parameter Kubelka-Munk method which describes an ink with one spectral parameter, (K/S)(λ), or by two spectral parameters, scattering S(λ) and absorption α(λ), where λ is the wavelength. See James: “Kubelka-Munk Theory and the Prediction of Reflectance”, Rev. Prog. Coloration, Vol. 15, pp. 66–75, 1985.
Determining the two colorant parameters involves measuring the spectrum on a bare substrate and on a black substrate, and solving the resulting equations. There also exist in the literature refinements on the two-parameter Kubelka-Munk theory that incorporate internal reflection, anisotropic scattering and other second order effects. However, these methods are applicable only for full (i.e., 100%) coverage of a layer of ink of a particular thickness, and/or are not applicable for variable ink coverages, for example, halftoning at less than 100% dot percentage. In addition, the colorant parameters they determine are neither substantially independent of the lowest substrate color nor are capable of incorporating dependencies on other ink layers. Thus these parameters are not easily applied to predicting the appearance of sequentially applied inks at less than 100% coverage wherein the substrate with the previously deposited inks may be regarded as a new substrate, with characterizable influence from layer to layer.
In summary, prior art techniques that are capable of accurately calculating overprints of inks, including rasterized (i.e., halftoned) inks, other than the techniques of the Parent Patents, require making a number of overprints of the colorants, so determining spectral parameters of a colorant that spectrally characterize the colorant. Furthermore, prior art techniques that spectrally characterize the colorants are not applicable to less than 100% coverage, do not produce parameters that are substantially independent of substrate color, and do not produce parameters that can take into account interactions between the colorant layers.
Thus there has been a need in the art for a method of characterizing colorants capable of accurately predicting the color of an overprint of the colorants without requiring measurement of overprints of the colorants and capable of predicting the color of overprints of halftones. These kinds of methods are the subject of the Parent Patents: U.S. patent application Ser. No. 09/361,435 and U.S. Pat. No. 5,933,578. One aspect of the present invention is extending these methods for use with device profiles.